1. Field of the Invention
The present invention relates to an analog signal buffer, and an image reading device using the analog signal buffer.
2. Description of the Related Art
Digital image forming devices such as digital copiers typically produce image data using the following method:    (1) irradiating an original document using an optical scanning system;    (2) converting light reflected from the original document to analog electric signals using a photoelectric converter such as a CCD image sensor (hereinafter referred to as a CCD);    (3) performing various analog processings on the analog electric signals; and    (4) converting the processed analog electric signals to digital data (i.e., performing an A/D conversion processing).
The processings performed on the analog electric signals and the A/D conversion processing are typically performed by an analog front end (AFE), which is an integrated circuit.
An output from a CCD is typically input to an AFE using an AC coupling, and change of the AC components of the output voltage is transmitted to the AFE. In this regard, it is necessary that the voltage input to the AFE is maintained so as to be not greater than the maximum rated voltage of the AFE, and it is satisfied as long as a normal operation is performed. However, when a power-on operation is performed, an excessive voltage is generated, and it is possible that the voltage input to the AFE becomes greater than the maximum rated voltage of the AFE.
FIG. 6 is a view illustrating change of the voltage output by a CCD when a power is supplied to the CCD. As illustrated in FIG. 6, when a power is supplied to the CCD, an output voltage (Vout) from the CCD exceeds a normal offset level of CCD and increases so as to be close to the supply voltage (Vccd) as the supply voltage (Vccd) for the CCD increases. The output voltage (Vout) rapidly drops so as to be close to the ground level (GND) when reset cancellation is performed. The mechanism of this voltage dropping is that charges, which are stored in the photoelectric converter of the CCD due to dark current therein before a power is supplied to the CCD, are discharged at a time when reset cancellation is performed. As illustrated in FIG. 6, after being dropped by the reset cancellation, the output voltage (Vout) is stabilized at the normal offset level.
When the output voltage from a CCD is thus elevated sharply as illustrated in FIG. 6, a risk of deteriorating the properties of the devices constituting the CCD or damaging the devices themselves seriously increases. In attempting to solve the problem, there is a proposal for an analog signal buffer which uses a delay circuit for delaying rise in the power supply voltage to prevent sharp increase of the voltage (Vout) output from a CCD to an AFE. By using such an analog signal buffer, damaging of the AFE caused by the excessive voltage of the CCD can be avoided, but generation of the reverse current caused by the above-mentioned dropping of the output voltage of the CCD cannot be prevented.
For these reasons, the present inventors recognized that there is a need for an analog signal buffer which prevents an AFE from receiving an excess voltage from a CCD while preventing the CCD from receiving a reverse current.